Much research has been carried out to improve water dissociating methods, there being much interest in producing hydrogen which is an easily stored non-polluting fuel with an inexhaustable raw-material source in the form of water. One way to dissociate water is the direct thermolysis of the water molecule by injecting steam into a reactor containing a refractory body raised to a high temperature (2,000.degree. to 3,000.degree. C.), in particular by focusing solar radiation onto it. As the water molecules make contact with this body, they are essentially dissociated into hydrogen and oxygen at a dissociation rate determined by the operating conditions.
However the implemention of this dissociation approach raises a difficult recovery problem for the high-temperature reaction products. As discussed by the article of E. Bilgen, M. Ducarroir et al in Int. J. Hydrogen Energy 2, (1977), pp 251-7, the chemical equilibria operative in the gas phase are such that slow cooling of this gas phase entails reagent recombination until there is total consumption of the products. Two solutions have been considered to overcome this problem.
In a first solution, following the dissociation, the hydrogen contained in the gas phase is separated at high temperature from the gas phase by means of a selective semi-permeable membrane. This method is discussed in the following publications: Iharas, Int. J. Hydrogen Energy, 3 (1978), pp 287-96; Fletcher. E & Moen R., Scienci, 197 (1977), pp 1050-6; Ounallia,. Cales B., Demrinski K & Baumard J.F., C.R Acad. Sci., 292, series 11 (1981), p 1185; Bilgen E., Int. J. Hydrogen Energy, 9, (1984),pp 53-58; U.S. Pat. No. 4,332,775, Battelle Memorial Institute.
Such a method however has several drawbacks. In the first place, semi-permeable membrane separation is comparatively slow and a portion of the decomposed gaseous products recombine before diffusing through the membrane, whereby the efficiency is significantly reduced. Further, there is present, before membrane diffusion, a high temperature gas phase in the reactor, including an explosive mixture which under some operational conditions may be dangerous. In particular such a procedure is poorly suited for continuous operation where focused solar radiation is used as the heat source for the refractory body, since the inevitable variations in incident energy entail a notable increase of the danger of explosion.
Another solution for the recovery of the decomposed products comprises rapidly chilling the gaseous mixture issuing from the reactor which then will be at a low temperature. The mixture no longer is reactive and the hydrogen extraction can be carried out by conventional separation means. Chilling may be implemented by injecting cold inert gases into the gaseous mixture, by contact with a cold surface or also by introducing the mixture into a chilling apparatus containing a low-temperature liquid. Information on this solution and its implementing techniques can be found in further detail in the following publications: Lede J., Weber C., Villermaux J., C.R. Acad. Sci., 286 (1978), p 299; Houzelot J.L. & Villermaux C., Chem. Eng. Sci., 32 (1977), 1465; Lapique F., Thesis, Inst. Nat Polytechnique, Nancy (France), 1983.
However this solution requires designing and adding an auxiliary chilling or cooling apparatus whereby the system is rendered substantially more complex and costly while its reliability is reduced. Moreover as regards chilling by cold gases, a gas flow is recovered which is greatly diluted in hydrogen and oxygen (less than 10% hydrogen), and the hydrogen and oxygen therefore are more difficult to isolate. Also, in most cooling equipment, the high-temperature gaseous mixture accumulates the reactor and in conduits upstream of the chilling apparatus, with the above mentioned difficulties (danger of explosion, unsuitability for continuous operation in the case of solar heating).
Accordingly a primary object of the present invention is to overcome these drawbacks of the prior methods and to provide a novel process and a novel apparatus for the dissociation of water by thermolysis.
Another object of the invention is to provide a dissociation process in which the thermolytically decomposed products are cooled and whereby the defects of the known chilling procedures can be overcome.
A further object of the invention is to carry out the chilling inside the very reactor in the immediate vicinity of the heated refractory body under suitable conditions to restrict to a maximum the recombination of the decomposed product and accumulation of the high-temperature reactive gas mixture, without employing an auxiliary chilling apparatus and without causing a gaseous dilution of the decomposed products.
Another object is to provide a dissociation apparatus of extremely simple design and reliable, dangerfree operation.
Another object is to achieve continuous operation with minimal danger, regardless of the heating mode of the refractory body and in particular for the preferred case of focused solar heating.